Task-dependency and structure-dependency in number interference effects in sentence comprehension

Franck, Julie; Colonna, Saveria; Rizzi, Luigi

doi:10.3389/fpsyg.2015.00349

ORIGINAL RESEARCH article

Front. Psychol., 10 April 2015

Sec. Psychology of Language

Volume 6 - 2015 | https://doi.org/10.3389/fpsyg.2015.00349

This article is part of the Research Topic Encoding and Navigating Linguistic Representations in Memory View all 49 articles

Task-dependency and structure-dependency in number interference effects in sentence comprehension

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Corrigendum: Task-dependency and structure-dependency in number interference effects in sentence comprehension
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$\r\nJulie Franck*$ Julie Franck^1*

Saveria Colonna² $Luigi Rizzi,\r\n$ Luigi Rizzi^3,4

¹Laboratoire de Psycholinguistique, University of Geneva, Geneva, Switzerland
²Centre National de la Recherche Scientifique – University of Paris 8, Paris, France
³Department of Linguistics, University of Geneva, Geneva, Switzerland
⁴Interdepartmental Centre for Cognitive Studies of Language, University of Siena, Siena, Italy

We report three experiments on French that explore number mismatch effects in intervention configurations in the comprehension of object A’-dependencies, relative clauses and questions. The study capitalizes on the finding of object attraction in sentence production, in which speakers sometimes erroneously produce a verb that agrees in number with a plural object in object relative clauses. Evidence points to the role of three critical constructs from formal syntax: intervention, intermediate traces and c-command (Franck et al., 2010). Experiment 1, using a self-paced reading procedure on these grammatical structures with an agreement error on the verb, shows an enhancing effect of number mismatch in intervention configurations, with faster reading times with plural (mismatching) objects. Experiment 2, using an on-line grammaticality judgment task on the ungrammatical versions of these structures, shows an interference effect in the form of attraction, with slower response times with plural objects. Experiment 3 with a similar grammaticality judgment task shows stronger attraction from c-commanding than from preceding interveners. Overall, the data suggest that syntactic computations in performance refer to the same syntactic representations in production and comprehension, but that different tasks tap into different processes involved in parsing: whereas performance in self-paced reading reflects the intervention of the subject in the process of building an object A’-dependency, performance in grammaticality judgment reflects intervention of the object on the computation of the subject-verb agreement dependency. The latter shows the hallmarks of structure-dependent attraction effects in sentence production, in particular, a sensitivity to specific characteristics of hierarchical representations.

Introduction

The wide literature on agreement in sentence production has given rise to a large body of research on the phenomenon of interference called ‘attraction.’ In the standard and most explored case, the speaker incorrectly produces a verb that agrees with a plural noun situated in a modifying prepositional phrase (PP) linearly intervening between the subject and the verb (e.g., *The time for fun and games are over, from Bock and Miller, 1991; Bock and Cutting, 1992). As experimental evidence accumulated, it has become evident that various types of syntactic elements have the potential to trigger interference, including adjuncts (Franck et al., 2004) and immediately preverbal objects (Fayol et al., 1994; Hartsuiker et al., 2001; Hemforth and Konieczny, 2003; Konieczny et al., 2004; Franck et al., 2006, 2010; Häussler, unpublished), but also and more intriguingly elements that are not situated between the subject and the inflected verb in the linear word string. Such cases of interference have been reported in object relative clauses as illustrated in (1a) (Bock and Miller, 1991; Franck et al., 2006, 2010), questions (Vigliocco and Nicol, 1998), as well as cleft sentences (Franck et al., 2006). The present study addresses the question of whether similar interference effects are detectable in sentence comprehension. In particular, the work aims to address three questions. First, do the critical constructs from formal syntax, i.e., intervention, intermediate traces, and c-command, which capture attraction patterns in agreement production (Franck et al., 2006, 2010), also play a role in the computation of agreement in sentence comprehension? Second, are the processes involved in the computation of agreement features the same in production and comprehension? Third do different experimental techniques in comprehension tap on distinct aspects of agreement computation in performance?

(1) a.Jean parle aux patientes [_RC que le médicament *guérissent.]

Jean speaks to-the-PL patients-PL that the medicine-SG *cure-PL.

‘John speaks to the patients whom the medicine *cure.’

b.Jean dit aux patientes [_CC que le médicament guérit.]

Jean tells to-the-PL patients-PL that the medicine-SG cures-SG.

‘John tells the patients that the medicine cures.’

The paper is structured as follows. We first present the syntactic configurations underlying object interference. We then turn to the role of the experimental task in agreement computation. Subsequently, we report three experiments exploring the role of syntactic configurations in object interference in sentence comprehension. The first two experiments use different methodologies to test the role of movement and intermediate traces: they contrast attraction in the minimal structural pair consisting of object relative clauses, involving movement (as in 1a), and the superficially similar complement clauses without movement (as in 1b). The third experiment tests the role of c-command by manipulating attraction from moved complex objects. These objects involve both a c-commanding DP and a purely preceding DP within a PP modifier whose respective effects on attraction are systematically assessed (e.g., Quelles patientes du médecin dis-tu que le juriste défend? Which-PL patients-PL of the doctor do you say that the-SG lawyer-SG defends-SG? vs. Le chirurgien de quelles patientes dis-tu que le juriste défend? The surgeon of which-PL patients-PL do you say that the-SG lawyer-SG defends-SG?).

The Role of Syntactic Structure in Object Interference

In a detailed exploration of object attraction in sentence production, Franck et al. (2010) tested various hypotheses with respect to the structural conditions underlying agreement errors. The starting point of their work was the finding that despite its close superficial resemblance to the object relative (1a), the sentence complement clause (1b) fails to trigger attraction. Whereas in (1a) patients is the moved object of the target verb cure used transitively, in (1b) it is the unmoved object of the main verb tells while the target verb is used intransitively.

In four additional experiments, the authors explored the role of properties that distinguish relative clauses from complement clauses. Argumenthood was found to play no role in attraction since objects that are not part of the argument structure of the target verb, as in extraction from clausal complements, trigger similar attraction as thematic objects (e.g., Voici les otages que le journaliste *apprennent qu’on a blessés; Here are the hostages-PL that the journalist-SG *learn-PL that someone injured), while objects in their canonical post-verbal position did not generate attraction either. Participle agreement triggered by the object in French was also found to play no role in attraction, since attraction effects were found with elements that fail to trigger participle agreement like accusative clitics in the causative construction (e.g., Le directeur les *font acheter; The director-SG them-PL make-PL *buy). Moreover, the strength of object attraction in structures in which the object has moved to the front of the sentence and fails to intervene linearly between the subject and the verb (relatives and clefts) was shown to be of a similar strength to that of a linearly intervening object, as is the case of the clitic object pronoun (e.g., L’avocat les *défendent; The-SG lawyer them-PL *defend, Franck et al., 2006). All these cases involve an object (or its trace) intervening in a c-commanding position between the terms of the agreement relation, the subject and the agreeing verb (see Franck et al., 2006, 2010 for a graphical illustration of the hierarchical structure and c-command relations). Evidence suggests that attraction is significantly weaker if the attractor intervenes purely in terms of precedence, i.e., in a position situated lower down in the tree and that fails to c-command the agreement node, as is the case of the modifying PP (e.g., L’avocat des patientes *mentent; The-SG lawyer of the patients-PL *lie-PL) or the dative clitic (e.g., L’avocat leur *mentent; The-SG lawyer to them-PL *lie, The lawyer lie to them, Franck et al., 2006). In sum, the empirical evidence suggests that object movement is a necessary and sufficient condition for object attraction to arise in an SVO language like French (conditions may differ for SOV languages if the object originates in preverbal position), and that c-commanding attractors generate more interference than preceding ones.

But why does the object interfere with agreement even in cases like (1a), in which it is pronounced in a position from which it does not intervene, either linearly or hierarchically, between the subject and the verb? Franck et al. (2006, 2010) noted that interference with the object seems to occur at a position that is neither the final nor the initial position since these two positions failed to generate attraction. The authors proposed that the intermediate position of the object in the hierarchical structure, mediating its initial position in the thematic structure and its final surface position, plays a crucial role. In a (much simplified) representation of example (1a) like, Jean parle aux patientes [_RC que le médicament t₂*guérissent t₁], the object patientes is initially generated in t₁ and then moves in t₂, a position which intervenes on the subject-verb agreement (AGREE) relation between the position hosting agreement morphology (ultimately attached to the verb) and the subject in its initial thematic position. Finally, the object moves higher to reach its final position. The intermediate position t₂, unpronounced but with morphological reflexes in some cases such as participial agreement in French (Kayne, 1989) or wh-agreement in Austronesian languages (Chung, 1998), is postulated in formal syntactic analyses for locality reasons (e.g., to respect Phase Impenetrability in a system like Chomsky, 2001; see Gibson and Warren, 2004 for experimental evidence for the role of intermediate traces in the processing of long-distance dependencies). So, we argued that formal characteristics of abstract representations assumed in the “principles and parameters”/minimalist analysis of agreement form the representational basis over which agreement processes operate in performance. The intermediate trace of the object in the vP periphery may be thought of, in a phase-based architecture, as corresponding to a temporary memory buffer from which the object remains active and available for further processes (Chesi, 2005). The activation of the object in memory in this precise position intervening on agreement would be the locus of the interference effects observed in agreement production.

Task-Effects in Object Interference

Production and comprehension critically differ in that whereas in production the speaker has access to the conceptual structure of the sentence, this structure is incrementally built in sentence comprehension, under the strong guidance of predictive mechanisms (see e.g., Hale, 2001; Levy, 2008). Nevertheless, under the view that the effects reported in agreement production reflect properties of the hierarchical structure over which agreement is calculated, one expects the same effects to show up in sentence comprehension (e.g., see Kempen et al., 2012, for a model with shared representations and shared processes of syntactic encoding and decoding). Various studies have shown that plural attractors situated in the subject phrase interfere with verb agreement processing in sentence comprehension (e.g., Nicol et al., 1997; Pearlmutter et al., 1999; Pearlmutter, 2000; Kaan, 2002; Thornton and MacDonald, 2003; Häussler, unpublished). The typical finding is that participants spend more time reading or judging the acceptability of a sentence in the presence of a plural mismatching subject modifier as compared to when it is singular.

However, only a few studies have explored object attraction in comprehension. Clifton and colleagues conducted two grammaticality judgment studies exploring attraction in structures like (2) (Clifton et al., 1999). They found that although participants correctly reject ungrammatical sentences like (2a), they tend to accept ungrammatical sentences like (2b) in which a plural element (people) is situated higher than the subject and the verb. Clifton et al. argued that the relative acceptability of (2b) lies in the fact that the chain between the moved element and its trace is still active, and the agreement dependency is on its path. The effect was attributed to the passing of the plural feature on the agreement dependency and not to a late gap complexity effect affecting the general difficulty in processing (2b), since the same sentences with a singular attractor (people was replaced by person) were systematically rejected, attesting that the difficulty selectively arises in the presence of the plural feature (see also Kayne, 2000 for the hypothesis that the agreement pattern in (2b) is fully grammatical in some dialects). The authors concluded that interference arises because of the link of the projection path that is shared by two distinct syntactic dependencies (agreement and the NP-trace chain). These findings and the overall interpretation are consistent with the analysis (just summarized) in Franck et al. (2006, 2010), which would further specify that interference would be triggered in (2b) by the transit of the plural relative head people in the periphery of the vP headed by think, a position from which people hierarchically intervenes between manager and the agreeing head, thus giving rise to the plural form think.

(2) a.Lucine dislikes the people who think the manager *know the answers.

b.Lucine dislikes the people who the manager (*)think t know the answers.

Using a methodology based on a two-choice response time paradigm, Staub (2009, 2010) also found evidence for interference from moved objects when participants were asked to select between the two verb forms (singular vs. plural) presented simultaneously on the screen after being exposed to the subject phrase one word at a time in a Rapid Serial Visual Presentation (RSVP) procedure. Verb selection was found to be sensitive to attraction similarly to sentence production, with slower response times to select the correct verb form in the presence of a mismatching plural feature on the object of the sentence. Note, however, that it is unclear whether the task taps into comprehension or production, since both components are involved.

Wagers et al. (2009) investigated object interference in two experiments on object relative clauses (Experiments 2 and 3) involving a self-paced reading procedure. The results showed a significant effect of the object number in the region following the critical verb. However, the effect was restricted to ungrammatical sentences and showed up in the form of faster reading times in the presence of a mismatching plural attractor, suggesting that it reflects a grammatical illusion lying in the incorrect computation of agreement. More generally, across the five self-paced reading experiments on both object and subject modifier attraction, the authors consistently found no attraction in grammatical sentences. This finding appears prima facie to be in contradiction with the other reports of significant interference in grammatical sentences involving subject modifiers, but the careful testing of these structures by Wagers et al. (2009) suggests that the attraction effect observed at the verb in these studies was actually a spillover effect from the plural feature preceding the verb. In line with that interpretation, they found that the introduction of an adverb between the modifier and the verb dissolved the effect (e.g., The slogan on the posters unsurprisingly was designed to get attention). Recent evidence from on-line experimental techniques further supports the view that attraction is restricted to ungrammatical sentences (eye-tracking: Dillon et al., 2013; ERP: Tanner et al., 2014). The possibility that attraction only arises in ungrammatical sentences in comprehension has important consequences for models of agreement computation. Wagers et al. (2009) suggest that attraction in sentence comprehension is driven by the properties of a cue-based retrieval process triggered when the parser encounters an agreement error: the system involves a predictive component by which the parser expects a particular number feature on the verb, and only if the bottom-up features of the verb mismatch the top–down predicted features is the cue-based-retrieval deployed to check whether the correct feature was missed during the first pass. On this view, number interference in comprehension arises from a fundamentally different cause from attraction errors in sentence production.

Nevertheless, as Wagers et al. (2009) point out, other studies reported reliable interference effects in grammatical sentences that cannot easily be explained by an extended effect of the plural attractor. In a series of five experiments using a Maze task (requiring for each upcoming word in the sentence to select amongst two words, and in the critical region, between a correctly agreeing verb and a word from a different grammatical category, e.g., was vs. ink) or a sentence classification task (requiring to determine whether the sentence is a legitimately ordered string of words), Nicol et al. (1997) found significant interference in grammatical sentences. Since in these tasks response times reflect either the selection of the correct grammatical category or the global assessment of the sentence word order, the interference found does not seem to be attributable to the spillover of the plural feature processing on the verb. In a speeded grammaticality judgment task on sentences containing an embedded clause with complex subjects modified by a genitive phrase, Häussler et al. (2003) reported interference from the plural attractor in grammatical sentences only; no interference was found in ungrammatical sentences. Using a similar procedure of grammaticality judgment, Häussler and Bader (2009) also found interference from a mismatching feature within a relative clause introduced by a possessive pronoun in both grammatical and ungrammatical sentences. Here again, the slow down observed in mismatch condition cannot be attributed to a spillover effect of processing the plural feature on the attractor linearly preceding the verb.

Summing up, while some studies of attraction in sentence comprehension point to similarities with sentence production, others suggest differences. Moreover, discrepancies are also found between studies of attraction in sentence comprehension, some reporting attraction in ungrammatical sentences only, others in both grammatical and ungrammatical sentences, and yet others finding attraction in grammatical sentences only. However, a direct comparison across these studies is difficult due to the fact that they involve different tasks and different syntactic structures. The role of the task in language performance gained increased interest in the recent years (e.g., Caplan et al., 2008; Caplan, 2010; see also Salverda et al., 2010 for an overview of task effects in the visual world paradigm), and it therefore seems crucial to collect finely controlled comparisons on agreement performance before conclusions be drawn with respect to the mechanism of attraction.

In order to test the potential influence of the task on attraction, Experiments 1 and 2 use the same materials tested in agreement production by Franck et al. (2010), but with two different experimental methods. Experiment 1 uses a similar self-paced reading procedure to that used by Wagers et al. (2009) but differs from it in that only grammatical sentences were introduced, to maximize the naturalness of the comprehension process and avoid any potential contamination from the presence of ungrammatical sentences. Experiment 2 uses a procedure of speeded grammaticality judgment on the ungrammatical versions of these sentences, with the aim of maximally promoting agreement computation in comprehension. Finally, since Experiment 2 only involved ungrammatical sentences, Experiment 3 manipulated grammaticality in order to assess its role in the same procedure of speeded grammaticality judgment used in Experiment 2.

Experiment 1: Object Interference in Self-Paced Reading

Experiment 1 explores the role of object movement, modulated by its number specification, in a maximally natural sentence comprehension task involving the self-paced reading of grammatical sentences. The same materials as in Experiment 1 from Franck et al. (2010) were used, involving a minimal contrast between a structure with object movement in an object relative clause (1a), and a structure without object movement in a complement clause (1b). The two structures are identical in surface order; the main difference lies in the selection of the main verb, which takes a single complement in (1a) (thus enforcing the analysis of the que clause as a relative modifying the object DP) and two complements in (1b) (thus giving rise to a sentential complement interpretation for the que clause). Interference is examined on the agreement of the verb in the subordinate clause (to cure). In the relative clause (1a), ‘patient(s)’ is the object of the target verb ‘cures,’ and is therefore assumed to transit via the intermediate position at the embedded vP periphery intervening on the subject-verb agreement relation (Franck et al., 2010). In the complement clause (1b), ‘patient(s)’ is the unmoved indirect object of the main verb while the embedded verb ‘cures’ is used intransitively. If the intervention of the intermediate trace of the moved object on agreement creates interference in sentence comprehension similarly to sentence production, a slow down is expected at the verb in the presence of a mismatching plural object as compared to a singular object in object relatives (1a), but not in sentence complements (1b). However, if attraction in sentence comprehension reflects a process of ‘rechecking’ triggered by an erroneous agreement, no interference is expected from the plural feature in either of the two structures.

Method

Participants

Seventy-two students from the University of Geneva, aged between 18 and 40, took part in the experiment. They received course credit for their participation. The experiment was approved by the ethics committee of the Department of Psychology of the University of Geneva and informed consent was obtained from all participants.

Materials

The experimental materials consisted of the 24 sentences used in Franck et al. (2010) incorporated in a 2 × 2 design crossing structure (relative vs. complement) and the number of the object (singular vs. plural). All subject head nouns were singular. Subjects and objects were all animate. Since the initial sentences ended with the target verb, two windows were added after the verb in order to measure potential spillover effects. These windows contained an adverb followed by a locative phrase. Each sentence was decomposed into 8 windows corresponding to phrases (content word + grammatical word if present). All test sentences were grammatical with respect to subject-verb agreement. Each sentence was followed by a yes/no comprehension question that probed participants’ interpretation of the thematic relations in the sentence. Examples of test items are presented in Table 1 (the full list of items is available in the Supplementary Materials).

TABLE 1

TABLE 1. Example of item in the four experimental conditions of Experiment 1.

An additional set of 48 grammatical filler items were built. Half of them had the same structure as the experimental materials (12 Object relative clauses and 12 Complement clauses) but with plural subjects (half with singular objects). The other half involved a variety of syntactic structures (eight declaratives, eight relatives, four temporal modifiers, four PP modifiers) with a varying number of reading windows.

Procedure

Sentences were presented on a computer screen using the e-prime software in a self-paced moving window paradigm (Just et al., 1982). Each sentence was first presented with dashes replacing words. Participants were instructed to read sentences by pressing the space bar in order to have the segments appear. Once read, windows disappeared from the screen such that only one window was readable at a time. Participants were told that they would also have to answer yes/no comprehension questions about the content of these sentences. Instructions encouraged both rapid reading and correctness in answering the questions. Order of presentation of the sentences was randomized. The experiment started with four practice trials.

Data analyses

Analyses of reading times were run after excluding incorrect responses to comprehension questions (181 data points rejected representing 10.2% of the data). Remaining response times were then trimmed for outliers, defined as data points with a value above 3 s for all participants and all regions (representing 3.1% over all responses). They were treated as missing values. Log-transformed response times and accuracy proportions were analyzed with (generalized) linear mixed-effects regression models with random intercepts for participants and items (Baayen et al., 2008), using the statistical software R (R Development Core Team, 2013). Estimates, t-values (for LME), z-values (for GLME) and p-values for the fixed factors and interactions were obtained via the lmerTest package, which provides p-values calculated based on Satterthwaite’s approximation. Significant interactions were further explored with (generalized) linear mixed-effects regression models separately on each of the two modalities of one of the variables involved in the interaction.

Results

Reading Times

The distribution of reading times across the different experimental conditions in the different regions is reported in Figure 1.

FIGURE 1

FIGURE 1. Distribution of reading times (in ms) in the four experimental conditions of the different regions of Experiment 1. The critical region containing the test verb is Region 6.

Region 2 (main verb). A marginal effect of structure was found with slower response times on the main verb of the relative clause condition (581 ms) than on the main verb of the complement clause (561 ms; β = -0.033, t = -1.76, p = 0.081). There was no effect of number and no interaction (ts < 1).

Region 3 (object NP). No main effect or interaction was significant (all ts < 1).

Region 4 (complementizer). No main effect or interaction was significant (all ts < 1).

Region 5 (subject NP). No main effect or interaction was significant (all ts < 1).

Region 6 (target verb). A significant effect of number was found, with slower response times for singular objects (695 ms) than for plural objects (623 ms; β = -0.078, t = -2.73, p = 0.006). The main effect of structure was not significant (t < 1), but it entered into an interaction with number (β = -0.013, t = -1.94, p = 0.053). Subsequent models exploring the interaction showed that whereas number significantly affected reading times in the relative clause condition (β = -0.115, t = -2.23, p = 0.027), it failed to affect them in the complement clause condition (t < 1).

Region 7 (adverb). A trend toward slower response times in the complement clause condition (671 ms) than in the relative clause condition (657 ms) was found (β = -0.029, t = -1.34, p = 0.180). There was no effect of number and no interaction (ts < 1).

Region 8 (locative). A trend toward an interaction between number and structure was found (β = 0.055, t = 1.17, p = 0.24). There was no effect of number or structure (ts < 1).

Accuracy

The distribution of mean accuracy scores in the four experimental conditions is illustrated in Figure 2. Generalized linear mixed effect analysis showed that accuracy was significantly higher in the complement clause condition (0.97) than in the relative clause condition (0.89; p < 0.001; β = -1.467, z = -3.89, p < 0.001). The interaction and the number effect were not significant (ts < 1).

FIGURE 2

FIGURE 2. Accuracy proportions in the comprehension questions of Experiment 1.

Discussion

Experiment 1 shows that the object’s number feature influences processing speed at the verb in the object relative clause condition, but not in the corresponding complement clause condition, despite the surface similarity between the two structures. The observed effect shows that a feature mismatch between the extracted object and the subject of the relative clause speeds up reading of the verb segment. As mentioned in the Introduction, the experimentally elicited production of agreement is made more error-prone by the presence of a feature mismatch. Under the hypothesis that number mismatch influences the computation of agreement similarly in production and comprehension, one would have expected to find that it slows down sentence comprehension processes. Rather, it appears that a feature mismatch makes production of the relative clause verb error-prone, but reading of the relative clause verb faster. Why does a featural mismatch trigger opposite results in production and comprehension? Three observations suggest an answer, which is that the comprehension task did not tap into the mechanism of agreement computation, but rather in a mechanism of chain resolution responsible for linking a moved element to its trace.

First, the present experiment shows an influence of the object’s number in grammatical object relative clauses, which contrasts with the study by Wagers et al. (2009) who only found an effect in object relatives that contained an agreement error. Wagers et al. (2009) suggested that the interference effect they reported reflects reanalysis: only if the verb feature conflicts with the predicted feature would a cue-based retrieval process be deployed to actively retrieve the matching feature in the parsed tree. Our finding of object interference in the context of naturally reading a grammatical sentence cannot be accounted for by this view that the mismatch effect only arises as part of a second-pass process of agreement ‘rechecking.’

Second, our finding that participants were faster in the presence of a plural feature mismatching the singular subject than in the presence of a singular feature matching the singular subject also contrasts with various comprehension studies showing a detrimental effect associated with a plural mismatching feature, whether it is on a subject modifier or a moved object, and whether comprehension is tested by way of self-paced reading or more indirect experimental procedures like maze tasks, classification tasks, grammaticality judgment or two-choice verb selection (Nicol et al., 1997; Clifton et al., 1999; Pearlmutter et al., 1999; Pearlmutter, 2000; Staub, 2009, 2010; Häussler, unpublished).

Third, although the direction of the interference effect in Experiment 1 may, at first glance, appear to be in line with that reported by Wagers et al. (2009) who also found faster reading times with plural mismatching objects, the two effects fundamentally differ since we found the effect in grammatical sentences whereas Wagers and colleagues found it in ungrammatical sentences. In the latter case, the parser is lured by the presence of a plural feature on the object creating an illusion of grammaticality. Hence, the finding that participants were faster in the plural mismatch condition also argues against a structure-based feature spreading mechanism like the one assumed to take place in production (Nicol et al., 1997; Vigliocco and Nicol, 1998; Franck et al., 2002; Eberhard et al., 2005). This raises the intriguing possibility that a different mechanism underlies the mismatch effect found here.

How do these different aspects of the data inform us about the mechanism underlying the number effect found in the present experiment? Directly relevant to the present study are the recent reports in the acquisition literature of intervention effects in the comprehension of object relatives. Adani et al. (2010) and Adani (unpublished) found that both English and Italian speaking children showed better performance in a sentence-picture matching task when the object and the subject of the object relative clause mismatched in number (e.g., Show me the elephant that the lions are washing is better understood than Show me the lion that the elephant is washing). Using a similar task Belletti et al. (2012) reported better performance in Hebrew-speaking children for sentences involving a gender mismatch between the subject and the object relative. Empirical evidence suggests that only features that play an active role as triggers of syntactic movement have the potential to influence comprehension. This conclusion was reached on the basis of cross-linguistic evidence showing that in contrast to Hebrew children, Italian children failed to show a comparable sensitivity to gender mismatch in their comprehension of object relative clauses: this property was related to the different syntactic status of gender agreement in Italian.

According to the version of Relativized Minimality (Rizzi, 1990, 2004) assumed in the references quoted (along the lines developed in Friedmann et al., 2009), what makes the relevant kind of object relatives problematic for children is the intervention of the subject DP in the path connecting the relative head and its trace in object position. In particular, the difficulty is attributed to the set-theoretic relation of inclusion that characterizes the feature make-up of the object and that of the subject. When both the object and the subject are singular, the object is endowed with features [+R, +N, +Sg] (where +R is the feature designating the relative head) whereas the subject is endowed with features [+N, +Sg]: hence, the featural make-up of the intervener is included in the one of the antecedent. Friedmann et al. (2009) proposed that inclusion is problematic for children to explain their difficulty with making the required connection between the object and its trace. However, if the subject is plural, the number mismatch creates an intersection set, the object carrying [+R, +N, +Sg] and the subject [+N, +Pl]. Intersection is higher than inclusion in a natural scale of distinctness, a relation that is assumed to be accessible to the child’s system in Belletti et al. (2012). Transposing this approach to the adult data collected in Experiment 1, the slowing down of reading time at the verb in the match condition as compared to the mismatch condition may be interpreted as an indication of the same gradation observed in children, inclusion being more difficult than intersection.

In this view, there is no contradiction between the number effect found in the production and comprehension studies of object relatives: whereas the production experiments directly tap into the agreement process, requiring the choice of a properly agreeing form, an operation which is penalized by the presence of a mismatching intervener in the immediate vicinity, the reading of the sentences primarily reflects the process of structure building, and in particular of the building of an appropriate A’-chain across an intervener, an operation which is enhanced by number mismatch. Hence the seemingly opposite consequences of mismatch in production and comprehension may be seen as a byproduct of the specific demands of the experimental tasks. If self-paced reading, as it is used in Experiment 1, mostly reflects the time taken by the parser to build the sentence structure and resolve the A’-dependency, it does not directly bear on agreement computation; one direct prediction of that account would be that the same effect of feature mismatch should be observed in sentences that do not involve an agreement configuration in their structure. We are currently exploring that possibility.

If self-paced reading does not tap into agreement processes, at least when complex sentences involving movement are involved, it may be relevant to identify a task that taps into the component of agreement processing in sentence comprehension. Experiment 2 uses a speeded grammaticality judgment task with sentences involving ungrammatical agreement, such that participants were forced to process agreement. If the same computational principles of agreement are at play in this task as in production, Experiment 2 should uncover the same structure-dependent attraction effects as found in sentence production.

Experiment 2: Object Attraction in Speeded Grammaticality Judgment

Experiment 2 tests the same experimental conditions as Experiment 1, contrasting object relatives and sentence complements, but this time with a speeded grammaticality judgment task in which agreement computation is explicitly assessed. Hence, in this task, agreement markers cannot be used for the structure building process; rather, agreement can only be computed once the hierarchical structure has been built. If the grammaticality judgment task allows tapping specifically into agreement processing in sentence comprehension, and if this process shows the signature of intervention effects as reported in sentence production, interference is expected to show up selectively in the condition where the object intervenes on the agreement dependency, i.e., in object relatives. In contrast, no interference is expected from the object of the main verb in sentence complements. Moreover, interference should take the shape of an attraction effect, with slower judgment times in the presence of plural mismatching objects.